As if on cue, a beautiful sun emerged on Sunday morning from behind clouds over the Thames as seven "extreme action" dancers dangled off the Millennium bridge to the delight of around 250 people.

Three hours later and a mile east, 10 times that number watched the choreographer Elizabeth Streb, 62, and two of her dancers make their way – slowly – down the outside of City Hall. It was heart-stopping to watch.

"I was a little terrified," admitted Streb. "But I was having a lot of fun."

The events were among seven guerrilla-style performances at London landmarks taking place as part of the London 2012 festival. Each was shrouded in secrecy until the power of social media brought out crowds that increased in number throughout the day.

The Millennium bridge event was – unsurprisingly since it was at 7.30am – the most sparsely attended. Small children were more intent on throwing pebbles, stones and – despite the protestations of parents – bricks into the low tide as the dancers spun on their bungee ropes in a performance called Waterfall. A solitary duck swam upstream, bemused by all the kerfuffle.

The dancers were choreographed precisely, their performance lasting for around 15 minutes until four support boats came in to get them down.

At City Hall, Streb and her two dancers performed Skywalk – an abseil down the outside of the Norman Foster-designed glass building on the Thames' south bank.

Streb admitted she did not have to do it herself, but said she wanted to thank the mayor's office for backing the London 2012 festival and give something back to the project, which is her company's biggest ever. "I hope this doesn't sound obnoxious but I wanted to pay tribute to the building and what it represents," she said.

Brooklyn-based Streb, who has been called the Evel Knievel of dance, blends her choreography with stunts, circus and sport. She performed her sky walk wearing a very striking pair of gold boots designed by John Fluevog. "I had been having nightmares about whether the boots would stick," she said. "Every time we went on to another piece of glass it was completely different. It was like an ice skating rink. Sometimes you put your feet ahead of you, sometimes you put your feet behind you and leave them there and just sink in to your harness and maintain your balance."

Streb spent two years combing London for the best locations for the festival. There were polite nos from some locations – the dome of St Paul's, for example – while others were too expensive, such as Tower Bridge.

After City Hall, the Streb bandwagon moved to Paternoster Square, just north of St Paul's, where eight dancers performed on the spokes of a 28ft aluminium wheel. Another four performances by Streb's American dancers – as well as some recruited and trained in London – were due to take place on Sunday evening.

Ruth Mackenzie, director of the London 2012 festival, said it was all about once-in-a-lifetime events. Certainly the capital is unlikely to ever see anything like Streb again.

• This article was amended on 15 July. The original referred to "guerrilla performances". This has been changed to "guerrilla-style performances"